Converter automatic gain control



Nov. 12, 1940. D. E. FOSTER 2,221,087

CONVERTER AUTOMATIC GAIN CONTROL FilecLOct. 11, 1938 70A. E NETWORK ATTORNEY.

Patented Nov. 12, 1940 UNITED STATES CONVERTER AUTOMATIC GAIN CONTROL Dudley E. Foster, South Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 11, 1938, Serial Noe 234,379

8 Claims.

My present invention relates to the automatic gain control of converter networks, and more particularly to the gain control of converter tubes of the type having a normally low input circuit 6 conductance.

One of the main objects of my invention is to improve the operation of a converter of the type having an oscillator section and a mixer section; the input circuit of the mixer tube being regu- 10 lated so as to maintain the conductance thereof substantially constant over a wide range of signal amplitude variation.

Another important object of my invention is to provide a converter tube of the type whose input circuit conductance is maintained substantially uniform regardless of signal amplitude; the tube having the signal grid and oscillator grid thereof varied in bias so as oppositely to vary said input circuit conductance.

Another object of my present invention is to provide an automatic volume control (AVC) network for a converter of the type which includes oscillator and mixer sections and whose signal input circuit has a normally low conductance value at low bias values of the signal grid bias, the control network acting to adjust the signal and oscillator grids in bias so as to maintain the said low conductance value while concurrently varying the conversion gain.

Still other objects of this invention are to improve generally the efliciency and operation of a converter tube of the 6K8 type, and more especially to provide an AVC' circuit for such a converter tube which will be reliable in function and 35. readily manufactured and assembled in a receiver. g

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention J. itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, 7

able amplifying stages; the amplified carrier energy is impressed, as at T, upon the tunable input circuit I-2 of the 6K8 type converter tube. The oscillator section K-G-P1 thereof has a tunable tank circuit 5-6, and the variable condensers may be simultaneously adjusted with the prior amplifier condensers for carrier frequency selection; this being denoted by the dotted line 5. The tank circuit 56 is tunable over a range of local oscillation frequencies differing from the signal carrier frequency range by the value of the intermediate frequency (LE). The mixer sectionof the converter tube comprises electrodes KGiG2.GoG4-P, and the I. F. output energy is developed in the I. l t-tuned output network8. r i r i The I. F. amplifier 10, say a tube of the pentode type, has its tuned input circuit 9 coupled to the output circuit 8. The amplified I. F. energy, and the energy may be amplified in additional I. F. stages if desired, is impressedon the second detector by network Hl2 resonated to I. F. The demodulated energy, whether at audio or video frequency, is amplified-further, and the amplified modulation is finally reproduced. The AVC function is provided by the AVG rectifier l3, and the latter may be of the usual diode type deriving its I. F. input energy from the circuit H. The direct current voltage output of rectifier l3 increases in value with the I. F. carrier amplitude magnitude.

The AVC bias is transmitted tothe signal grids of the controlled tubes by the AVC lead IS. The latter includes a filter M for suppressing pulsations inthe bias. Lead 15 is connected to the low potential .end of coil through filter resistor 16 thereby providing the gain control connection to the signal grid Gs of the converter. Lead I5 is, moreover, connected to the signal grid of I. F. amplifier l0 through filter resistor it; a blocking condenser 20 connecting the low potential end of the circuit 9 to the cathode end of grounded resistor H. The. space current of tube I ll flows throughthe latter I. F bypass condenser l8 shunting the resistor.

The numeral 3 denotes the voltage supply bleeder resistor usually employed in areceiver; the negative terminal thereof is at ground potential. The cathodeK ofthe converter tube is connected to an intermediate point 4 on the resistor 3; condenser 4 bypasses the cathode to ground for radio frequency currents. The oscillator plate P1 is connected to a desired positive potential point (shown as +3) on the bleeder resistor 3 through tank coil t; the blocking condenser 6' connecting the grounded end of variable condenser 5 to the positive terminal of coil 6. The oscillator grid G is connected by the condenser 23 to the high potential end of the feedback coil 1; the latter is reactively coupled to tank coil 6.

Local oscillations developed by the oscillator section of the converter tube are impressed upon the grid G1. The grids between the cathode K and the plate P are arranged in succession in the electron stream therebetween and the grid G1, or mixer grid, is connected by lead 2| to the cathode end of resistor II. The lead 2i includes the filter resistor 22 for suppressing pulsations in the direct current voltage applied to the mixer grid G1. The screen grids G2 and G4 are maintained at a common positive potential, and this potential may be the same as the potential applied to the plate P. In order to have the functioning of the present invention clearly understood, the following explanation of the converter tube construction is given. The 6K8 type of tube functions to produce the I. F. energy primarily by modulation of the mixer unit transconductance by the oscillator voltage. The converter tube is essentially a triode-hexode wherein the oscillator and mixer elements are disposed in separate electron streams, The 6K8 type tube gives improved performance in substantially all frequency bands, but its performance is most spectacular in the high frequency regions above 15 mo. regions interlock is very greatly reduced; and loading of the signal input circuit is not only reduced, but by the proper choice of operating conditions may be made negative in sign.

To explain the reason for the negative input conductance of the signal input circuit in a 6K8 type tube, it may be explained that the space current between G1 and G2 is relatively independent of the bias on signal grid G3, due to the interposed screen G2. In other words, the sum of the currents in the mixer plate P and the screens G2 and G4 is relatively constant. The electrons passing through G2 will be subject to the'influence of G3 and at some high negative bias on G3 will be completely prevented from passing through this grid, resulting in mixer plate current cut-off asin the case of the ordinary triode. However, as the negative bias on G3 is decreased from some high value the mixer plate current will not increase linearly, because the supply of electrons going through G2 is limited. This limitation is brought about because the potential on the mixer grid G1 exerts the primary influence for determining how many electrons shall be available for the remainder of the mixer section. l

The grid G2, by virtue of its potential and its location, will draw some value of current, which current will be relatively independent of the potential of G3, with the remainder of the mixer section current passing to plate P, Therefore, by a proper choice of potentials in G1 and G2 there will be a scarcity of electrons on the G3 side of G2 for some range of potentials on G3 (low values of negative bias). Then, With electrode potentials selected to cause an electron deficiency in the Gz'G3 space over a certain range of bias values on G3, it may be said to a first approximation that the current passing through G3 is constant with respect to the potential of G3 over that restricted range of operation. If the signal grid potential increases, the velocity of the electrons in the space between G2G3 will increase and the charge will decrease. This de- In these.

crease in charge in the Gz-Ge space will cause electrons to flow into the signal gr-id, or current to flow out; thus, an increase in potential of signal grid G3 is accompanied by a change of opposite sense in the current in that electrode, which condition is the criterion in negative conductance.

The net input circuit conductance of the 6K8 tube will depend in sign and in magnitude upon the relative importance of the two efiects operating simultaneously within the G2G3 space. When the 6K8 is operated with a high value of negative bias on G3 the mixer plate current will decrease and the screen grid current will rise. These efiects will be accompanied by the building up of a space charge in the G2G3 region. Hence, at high values of negative bias on the signal grid the signal input circuit has a positive, or comparatively low negative, input conductance. It can also be shown that maximum negative conductance is attained in the signal input circuit when the tube is operating at a low signal grid bias and at a low oscillating strength. Since the radio frequency gain and the image ratio are greatly improved with maximum negative input conductance for the 6K8 tube, it will be appreciated that it is highly desirable to maintain said conductance value during receiver operation.

However, when utilizing the AVG circuit for automatically increasing the negative bias on signal grid G3 as the I. F. carrier amplitude increases, so as to maintain the carrier amplitude at the detector input circuit 12 substantially uniform, the negative input conductance of the converter tube actually decreases as explained previously. Hence with increase of AVG bias, the gain and image ratio of input circuit l2 are seriously impaired. Now, as explained previously, a reduction in the oscillator strength to a low value has the effect of increasing the negative input conductance of the tube, and if the oscillator strength is decreased to a magnitude less than a certain value, the conversion gain is also decreased. Hence, the oscillator grid G, as well as the mixer grid G1, isconnected by lead 2| to a circuit which functions automatically to in crease the oscillator grid bias as the carrier amplitude increases. If resistor 22 is the oscillator grid leak resistor connected to the bias resistor H, the AVG bias applied to the signal grid of tube It) causes the voltage drop across resistor H to decrease.

The oscillator grid G will become more negative, in this case, with respect to cathode K, which isbiased by the portion of the bleeder resistor 3 between point 4 and ground. In other words, asthe AVC bias increases, the oscillator grid G becomes biased in a negative polarity sense with respect to point 4 on bleeder resistor 3. This increase in bias of oscillator grid G reduces oscillati'on amplitude, and causes an increase in the negative input circuit conductance. Hence, it

. will be seen that as the AVG bias increases, the input conductance of circuit 1-2 will tend to remain substantially constant at a predetermined low, or negative, value by virtue of the opposing effects of the biasing of grids G3 and G. Therefore, the gain and image ratio remain substantially uniform over the entire range of AVG bias variation. Additionally, it is to be noted that the increase in AVC bias on the signal and oscillator grids causes a decrease in'conversion gain thereby improving the AVG action. The bias from the bleeder 3 and the amplified AVC voltage developed across resistor I1, and the magnitude of D. C. built up by oscillator action across resistors 22 and I! in series, are to be proportioned so that the input conductance is maintained substantially constant despite AVC. This cannot be done over the entire'characteristic of G3, but operation should preferably be confined to that operating region where the input conductance can be maintained substantially constant. There are a sufilcient number of variables, namely os-' resistor 11, to be able to obtain the desired result.

While I have indicated and described a system for carrying my invention into eiTect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What'I claim is:

1. A method of operating a converter tube of the type provided with oscillator and mixer sections and whose mixer section has a normally negative input conductance, which includes impressing signals and oscillations on the mixer section, and decreasing the oscillation amplitude sufliciently to introduce an input conductance variation opposed to any variation in the nega tive conductance caused while decreasing the mixer section gain thereby to maintain the said conductance substantially constant.

2'. In a converter tube of the type having oscillator and mixer sections, a signal input circuit connected to the mixer section, the potentials and electrode arrangement of said sections being such as to impart a negative input conductance value to said input circuit in the absence of signals, means for decreasing the mixer gain as the signals increase in amplitude, and additional means for decreasing the oscillation amplitude so as to' maintain said negative conductance value substantially constant.

3. In combination with a signal input circuit, a tube of the type provided with independent triode and hexode sections, said input circuit being connected between the input electrodes of the hexode section, a local oscillation circuit coupled to the triode section electrodes, said input circuit having a negative conductance value for a low potential difference between said input electrodes, an automatic volumecontrol circuit connected to said hexode section to increase said potential difference, and means responsive to said control circuit for decreasing the oscillation amplitude thereby to maintain said negative conductance Value substantially constant.

4. In a converter tube of the type having oscillator and mixer sections and whose input conductance is negative for low signal grid bias values and low oscillation amplitude, the method which includes increasing said signal grid bias to decrease conversion gain thereby varying the conductance sign and magnitude, and simultaneously decreasing the oscillation amplitude at said oscillator section thereby to produce a conductance variation opposed to the said first variation.

5. In combination with a tube having a cathode and two plates arranged to receive independbetween, the cathode and the signal grid, an oscillation voltage connection between said oscillator grid and said mixer grid, a positive screen grid between, the mixer grid and signal grid, a signalinput circuit connected between the signal grid and cathode, a local oscillation network coupled to the cathode, oscillator plate and oscillator grid, said mixer, screen and signal grids being at potentials such as to provide a low conductance for said signal input circuit, and means for varying the biases of the oscillator and signal grids to maintain said conductance while substantially varying the intensity or said streams.

6, In combination with a tube having a cathode and two plates arranged to receive inde pendent electron streams, a control grid in one stream. to provide with one plate an oscillator section, a signal grid in the other stream, a mixer grid between the cathode and the signal grid, an oscillation voltage connection between said oscillator grid and said mixer grid, a signal input circuit connected between the signal grid andcathode, a local oscillation network coupled to the cathode, oscillator plate and oscillator grid, said mixer and signal grids being at potening the intensity of said streams.

7. In combination with a tube having a cathode and two plates arranged to receive independent electron streams, a control grid in one stream to provide with one plate an oscillator section, a signal grid in the other stream, a mixer grid between the cathode and the signal grid, an oscillation voltage connection between said oscillator grid and said-mixer grid, a signal input cir cuit connected between the signal gridand cathode, a local oscillation network coupled to the cathode,oscillator plate and oscillator grid, said mixer and signal grids being at potentials such as to provide a low conductance for said signal input circuit, and means for varying the biases of the oscillator and signal grids in the same polarity sense to maintain, said conductance while substantially varying the intensity of said streams. i

8. In a converter tube of the type having oscillator and mixer sections and whose mixer section has a signal input circuit whose input conductance is relatively low for low signal grid bias values and low oscillation amplitude, the method which includes adjusting said signal grid bias to decrease the conversion gain thereby varying the conductance sign and magnitude, and simultaneously adjusting the amplitude of the oscillations produced by said oscillator section thereby to produce a conductance variation opposed to the said first variation. a

, DUDLEY E. FOSTER. 

